The future LTE-A (Long Term Evolution Advanced) system will support a transmission bandwidth up to 100 MHz, while the maximum transmission bandwidth supportable by the LTE (Long Term Evolution) standard is 20 MHz. Thus to achieve the higher transmission bandwidth, it requires to aggregate multiple carriers. Carrier aggregation (CA) is a technique of aggregating multiple carriers for combined transmission, which is proposed by 3GPP (3rd Generation Partnership Project) to meet the high transmission bandwidth requirement of the future mobile systems. Carrier aggregation may be classified into consecutive carrier aggregation and non-consecutive aggregation based on the positions of the carriers that are aggregated on the spectrum. LTE-A will support both of the two CA scenarios. While introducing the CA technique, 3GPP also considers its backward compatibility, which means that user equipments (UEs) supporting CA and UEs not supporting CA will co-exist for a long time in the future. A CA supporting UE can be connected to a plurality of component carriers (CCs) at the same time, and a UE not supporting CA can be connected to only a certain CC.
With the introduction of the CA technique, each cell may be configured with a plurality of CCs and a UE may also use multiple of CCs. Not all the UEs use all the CCs of the corresponding cell. Those CCs used by the UE are referred to as configured CCs and those not in use are called as non-configured CC. The so called configured/non-configured is defined with respect to each UE. That is, different UEs may have different configured/non-configured CCs
The configured CCs may be further classified as activated CC and deactivated CC. The UE transmits data over the activated CC. No data transmission is performed over the deactivated CC. The deactivated CC does not support difficult measurements such as CQI (Channel Quality Indicator).
The advantage of introducing activated/deactivated CC lies in that, the CCs that are not in use temporarily can be set in the deactivated state so that the power of the UE may be saved. The deactivated CC may be switched into the activated state rapidly via MAC signaling, and unlike the non-configured CC, the deactivated CC can perform measurement thereon and the measurement information of the deactivated CC can be used by the base station to set related parameters. In this way, the requirements of the burst data services can be better met.
In LTE-A, each cell has a plurality of CCs and each UE can be allocated with a plurality of CCs. LTE-A defines the concept of “special cell”. Each UE has only one special cell and the special cells of different UEs may be different from each other. Over the special cell, the system provides security input function and NAS (non-access) layer information to the UE. From the view point of the system, each CC is equivalent to a cell, and is allocated with globally unique cell identification. From the view point of the UE, even if the UE is allocated with a plurality of CCs, it can only see one cell, i.e. the special cell, while the other CCs are used as uplink and downlink resources.
LTE-A introduces the concept of PCC (primary component carrier). Each UE is configured with an uplink PCC and a downlink PCC. The uplink control information is transmitted over the uplink PCC. The radios link failure (RLF) information is monitored over only the primary cell (Pcell). The cell corresponding to the PCC is thus called as primary cell (Pcell), and the other cells are called as secondary cell (Scell). Pcell is the special cell.
To save the power of the mobile station, LTE Rel-8 introduces the concept of DRX (Discontinuous Reception) such that the listening of PDCCH (Physical Downlink Control Channel) is stopped when there is no data transmission over the air interface, thereby reducing the operation of the receiver, decreasing the power consumption of the mobile station and lengthening the life of the battery.
Some concepts of DRX in LTE Rel-8 are explained blow.
1. On Duration time: a UE wakes from dormant state and goes into the on duration, and starts an on duration timer. During the on duration, the UE receives the information of PDCCH. On PDCCH, there is signaling information related to the UE which is transmitted from the network side to the UE, such as control information of resource allocation, such as confirmation, power control, resource allocation and reallocation, etc. If the UE can successfully decode the information of the PDCCH signal indicating the initial uplink or downlink user data transmission, it starts a discontinuous reception inactivity timer and goes into the inactivity time; otherwise, the UE goes into the dormant state after the on duration time ends (i.e. expire of the on duration timer)
2. Inactivity time: after the UE successfully decodes the PDCCH channel, the UE starts the DRX inactivity timer and goes into the inactivity time. During the inactivity time, the UE continues listening to the PDCCH and related control channels. If the UE successfully decodes the PDCCH and related control channels before the expire of the DRX inactivity timer, the UE restarts the DRX inactivity timer and once again goes into the inactivity time; otherwise, the UE goes into the dormant time after the expire of the DRX inactivity timer, and proceeds to the next DRX cycle.
3. Active time: during the active time, the UE monitors the PDCCH channel; the on duration time and the inactivity time both belongs to the active time.
4. Dormant time: the UE is in an off state during the dormant time.
5. HARQ RTT Timer (Hybrid Automatic Retransmission Request Round-Trip Time): this timer is used for the timing of the least time interval predicted to be used for downlink retransmission of the UE. When a new downlink data transmission is detected, the HARQ RTT Timer is started, and if the received data is correctly decoded upon the expiring of the HARQ RTT Timer, the UE goes into the dormant time and proceeds to the next DRX cycle.
6. DRX retransmission timer: this timer is used for the timing of the time predicted to be needed for downlink retransmission for the UE. When the HARQ RTT Timer expires and there exists data that has not been successfully decoded in the corresponding HARQ buffer, the DRX retransmission timer is started, and this time the PDCCH is listened to.
7. Contention Resolution: once the uplink message contains C-RNTI (Cell Radio Network Temporary Identifier which is allocated by wireless network controller) MAC control element or the uplink message contains CCCH SDU (Common Control Channel Service (CCCH) Data Unit (SDU)), the UE starts a contention resolution timer and monitors the PDCCH until the contention resolution timer expires. When receiving the massage indicating the successful contention resolution, the contention resolution timer is stopped.
8. DRX short cycle timer: when the DRX inactivity timer expires, the DRX short cycle timer is started. If the DRX short cycle timer expires, a long cycle DRX is started. The UE can be configured as short DRX cycle and long DRX cycle. The short DRX cycle is optional. In the case that the short DRX cycle is configured, after entering into the short DRX cycle state, the UE goes into the long DRX cycle if it does not listened its own PDCCH packet after the DRX short cycle timer expires. If the short DRX cycle is not configured, the UE directly goes into the long DRX cycle.
If a DRX MAC (Media Access Control) control information unit is received, it means that the base station requires the UE to go into the dormant state. At this time, the on duration timer and the DRX inactivity timer are stopped, but the time related to the retransmission is not stopped.
With the introduction of CA, a mobile station can simultaneously use a plurality of CCs, which makes the DRX operation environment more complex. In addition, under CA scenarios, the mobile station and the protocol design face the huge challenge of high power consumption. How to make the DRX, which is an important means for saving the power of the mobile station, effectively work under CA scenarios is another issue to be solved.